A theoretical study of the potential energy surface and rate constant for an O(3P) + HO2 reaction

The potential energy surface for the reaction O(P-3) + HO2 --> HO3 --> HO + O-2 and the kinetic rate constant were investigated using ab initio calculations with the multireference Moller-Plesset second-order perturbation (MRMP2) method and dynamics calculations based on the microcanonical variational theory. The MRMP2 potential energy surface is compared with those of MP2, QCISD, and B3LYP based on a single reference wave function to characterize the HO3 molecule, which has a planar geometry. The MRMPT2 level of theory predicts that the trans geometry will be more stable than the cis geometry, while the MP2d, QCISD, and B3LYP levels of theory predict that the cis geometry will be more stable. This disagreement is due to the differences in the estimations of the central O-O bond strength at various levels of theory. The temperature dependence of the microcanonical variational rate constant, which was determined from the barrierless attractive potential energy surface in the entrance channel, is shown to agree with the data recommended by NASA. The potential energy surface, corrected for basis set superposition errors, is also used to estimate the lower limit of the rate constant at this level of theory. The contribution of direct hydrogen atom abstraction reaction to the rate constant is small at low temperature. However, the reaction is preferred over the reaction channel via HO3 at the high temperatures, especially above 1000 K.